Many satellites currently in operation were designed with a finite amount of propellant and were not designed for the possibility of being resupplied with propellant. The design philosophy relied upon replacement of the satellites after they had exhausted the on-board propellant supply. In view of the expense of replacing satellites, it would be very advantageous to be able to resupply satellites with propellant which are either near their end of propellant life but otherwise functional, or have suffered an infant propulsion system failure or insertion anomaly, or have been maneuvered more than originally intended for their nominal operations, thereby extending their operational life by several or many years.
It is estimated that as many as half of all GEO communication satellites end their 10 to 15 year life with all or most of their subsystems still functional and it is only the depletion of the carefully budgeted propellant load that drives retirement of the satellite. Using a current economic model, the ability to refuel several of these end-of-life satellites in a single mission would cost-effectively extend each of their useful lives by 3 to 5 years and thereby delay the need to outlay the very high capital costs to launch a replacement for each satellite if desired. Some satellites suffer from primary propulsion system failures or launch vehicle upper stage related failures soon after they are launched. In these cases the entire book value must be written off and compensation paid to the operator by the insurer. The satellite becomes an asset of the insurer and will eventually have to be disposed of in a graveyard or re-entry orbit. If these assets can be resupplied with propellant, enabling them to transfer to an orbital station in geosynchronous orbit and extending their life by 5 to 10 years, most or all of the value of the satellite can be recovered.
The key technical difficulty is that these satellites were not designed for robotic servicing, and it is not generally accepted that such missions are technically possible. Specifically, most satellites are designed with propellant fill and drain valves that were intended to be filled once prior to launch and never opened or manipulated following launch. Thus, accessing these fill and drain valves remotely in-orbit presents several major challenges and would involve several operations, each of which is difficult to accomplish robotically including: cutting and removal of the protective thermal blankets, removal of several lockwires hand wrapped around the valves, unthreading and removing outer and inner valve caps, mating a propellant fill line to the valve nipple, mechanically actuating the valve and, when propellant resupply is complete, replacing the inner valve cap.
On-orbit servicing has been the subject of much study over the past thirty years. The idea of maintaining space assets, rather than disposing of and replacing them, has attracted a variety of ideas and programs. So far, the concept has only found a home in the manned space program where some success can be attributed to the Solar Max and Hubble Space Telescope repair missions, Palapa-B2 and Westar rescue missions, and the assembly and maintenance of the International Space Station.
Until recently there have been no technologies disclosed that can solve the problem of accessing the propellant system of an unprepared satellite for the purpose of replenishing station-keeping propellant. The majority of satellites in orbit today were not designed with orbital propellant resupply in mind and access to the propellant system is designed to be accessed by a human on earth before launch. The technologies required to access the client spacecraft's propellant system for the purposes of propellant resupply still have a very low technology readiness level, and are generally considered to be the main obstacle to a successful servicing mission.
Transferring propellants used for spacecraft propulsion systems from one source to another can be very dangerous due to the corrosive and explosive nature of many of the fluids involved. For example, inadvertent mixing of fuel and oxidizer in bipropellant systems will cause immediate combustion, so a fluid transfer system for bipropellant needs to ensure that no accidental mixing occurs.
Therefore, it would be very advantageous to provide a propellant transfer system for transferring propellant from a servicing spacecraft to a client satellite which has flexibility to deliver propellant using more than one modality depending on the circumstances of the satellite, propellant system parameters, and the like. It would be very advantageous for such a system to able to able to transfer bipropellants in addition to monopropellants, pressurants, and ion or plasma propulsion propellants.